Full disclosure: I haven't made the plunge to X3 yet. Still on X2 Producer here.
I want the same features I've wanted since X1:
(01) Pagination of the Control Bar, so I don't need to buy a larger computer monitor and still have access to all the modules. Whoever decided that "module hidden" was a satisfactory solution to the screen real-estate issue has apparently NEVER used a computer before. I just want on-screen access to the basic tools again, like I had in 8.5.
(02) A new narrow strip layout in the Console View that gives us back all of our buttons that we'd have in wide strip view (e.g., polarity invert etc.).
(03) Driver aliasing that is actually "friendly" in that I can name discrete mono channels anything I want, and also can configure the layout of the drivers how I choose. I realize this isn't a flashy feature, but it will save headaches and reduce routing errors.
(04) Updated implementation for the MCU, Mackie XT and Mackie C4. Sonar is still the best I've come across for these devices, but they're getting increasingly out-of-date. Particularly with the emphasis on ProChannel, which isn't properly implemented in this regard. That basically makes ProChannel modules BROKEN PLUGINS from the perspective of somebody who relies on the MCU/XT/C4 (i.e., me).
(05) ALL ProChannel plugins bundled with Producer. The whole point of buying Producer for me that so I don't have to worry about feature cross-compatibility when working with other Sonar users. Making us buy ProChannel plugs a la carte effectively KILLS that advantage. I'd even be willing to pay a little more for a "ProChannel Expansion" version of Producer.
Right now for me, Sonar upgrades will typically hinge on 1-2 good features more than anything. With X1 it was screensets, and with X2 it was automation modes (smartsnap looked good, but in practice turned out to cause me more headaches than it cured). I have waffled on X3 because it doesn't seem to have any features that strike me as "must-have", though I DO like to have the most recent version so I can stay supported.
If the bakers are reading this, take note: I will buy the next TWO versions of Sonar Producer if in the next version you include #1-3. But apparently the feature request submission isn't getting lit up with these, despite the fact that it's a little embarassing that Sonar still has these pretty basic flaws.